Sometimes you design a perfectly good experiment based on years of experience and a wealth of previous data. You develop some models and carry out simulations that show you’ve designed an experiment that can recover your models. This lets you write a really compelling proposal and – eventually – you have the opportunity to carry out that experiment. And then the results surprise you. Because simulations are just that, simulations: they’re only as good as the physics you know to put in. You can’t account for what Donald Rumsfeld famously called the “unknown unknowns”. We found things we weren’t expecting, but on the other hand we found some things we were expecting and learned some new things as well.

My example is a spectroscopic monitoring program that was undertaken with HST in Cycle 21 with the goal of probing the inner structure of an active galactic nucleus (AGN, often called quasars, when they’re luminous enough). Together with its ground-based counterpart, this program is known as the AGN Space Telescope and Optical Reverberation Mapping (AGN STORM) project. Our goal is to understand how the supermassive black holes at the centers of galaxies are fueled.

The current paradigm for AGN inner structure (Figure 1) is that at the center of these systems is a central supermassive black hole (typically a million to several billion solar masses) surrounded by a hot accretion disk that extends out to tens of gravitational radii (Rg = GM/c2, where M is the black hole mass). On scales of a few hundred to several thousand gravitational radii, there is diffuse gas that absorbs the ionizing radiation from the accretion disk and reprocesses it within minutes into emission lines. The emission lines are strongly Doppler broadened because they are in the deep gravitational potential of the black hole. The geometry and kinematics of this “broad-line region” (BLR) remain elusive since these properties cannot be deduced from direct imaging as they project to less than 100 mas (milliarcseconds) even in the most favorable cases. What we know about the inner structure of AGNs is based on flux variability.

Figure 1. Classic schematic of the inner structure of an AGN from Urry & Padovani (1995). Here I restrict attention to the central black hole, surrounding accretion disk, and the broad-line region.

The continuum radiation from the accretion disk varies with time (as I’ll describe elsewhere) and the broad emission lines respond, but with a delay due to the mean light travel time across the BLR. This is the basis of the technique known as “reverberation mapping” – the emission lines appear to “reverberate” in response to the changing continuum, and measurement of the timescale can be converted to a size estimate (for a technical primer on reverberation mapping, see Peterson 1993). While gas is spread throughout the BLR, the response of any particular emission line is relatively localized to where some combination of emissivity (photons emitted per unit volume) and responsivity (rate of change in emissivity per unit continuum change) are maximized for that line. At any given time, the highest-ionization lines respond more rapidly than lower-ionization lines, demonstrating ionization-stratification of the BLR. For any given emission line, the radius at which the peak response occurs depends on the mean continuum brightness: the peak response occurs at longer lags when the AGN is brighter (Figure 2). What makes this interesting is that if you compare the measured lags with the line widths, you find that the Doppler width ΔV is inversely correlated with the time lag τ (Figure 3), consistent with ΔV ∝ τ -1/2, which is what you’d expect if the dynamics of the BLR are dominated by the gravitation of the central black hole – strictly speaking, it implies a 1/r2 force, so radiation pressure will have the same signature, but that’s a detail we can worry about later. In any case, without knowing the net motion of the BLR – which could be inflow, outflow, rotation, or mostly likely some combination of all of these – we can construct a “virial product” ΔV2cτ/G that is proportional to the black hole mass. Actually getting the mass, though, requires knowing more about the structure and kinematics of the BLR, as well as its orientation. In the absence of this knowledge, we parameterize our ignorance into a single dimensionless parameter f defined by M = f × ΔV2cτ/G. If, for example, the BLR is a simple flat disk (it’s not…) lags are insensitive to inclination, as long as the emission-line photons aren’t absorbed within the disk, and orbital velocities project as sine of the inclination i, so f = 1/sin2i. Our goal is to determine the structure and kinematics of the BLR, which is equivalent to knowing f and thus M for a particular AGN. It turns out this is hard.

Figure 2. The relationship between the size of the broad-line region as measured from the Hβ emission line and the luminosity of the AGN (Bentz et al. 2013).

Figure 3. The relationship between emission-line Doppler width and reverberation lag for multiple emission lines in four AGNs. The ΔV ∝ τ -1/2 dependence is expected for a system dominated by the gravity of the central black hole. The dashed lines are the best fits to the data, and the solid lines have a forced slope of -1/2. Based on data from Peterson & Wandel (2000) and Onken & Peterson (2002).

Reverberation signals are quite weak: over the BLR light travel time, the continuum and emission-line fluxes generally vary only a few to several per cent, at most. Over many light travel times, the flux variations can be larger, 10% or more. This tells us right away that we’re going to need high signal-to-noise, homogeneous spectra that are well-sampled in time over a long duration. This also tells you something about why progress in reverberation mapping has been slow – it requires a lot of telescope time. Consequently most reverberation experiments are carried out on relatively small telescopes on apparently bright, relatively nearby AGNs. Even then, the data are generally of insufficient quality to discern the structure and kinematics of the BLR, so the factor f remains undetermined. We can, however, compute an ensemble average value for f if we have another mass indicator that we trust. The one we have been using is the M- σ relationship, the apparently tight correlation between central black hole mass and the stellar velocity dispersion of the host galaxy bulge σ, that has been found for non-active galaxies. If you plot the virial product versus σ for AGNs, you see a relationship that is parallel to the M- σ relationship, and if you multiply the virial product by a factor of 4 or 5, the two relationships are indistinguishable (Figure 4). Thus < f > ~ 4 – 5. There are only a few AGNs where the black hole mass can be measured directly by stellar dynamics, and these show consistency with the reverberation estimates to within the uncertainties of around a factor of 3 or so.

Figure 4. The relationship between black hole mass and host galaxy bulge velocity dispersion, known as the M- σ relationship. The red points are for quiescent (non-active) galaxies and the blue and green points are for AGNs. From Grier et al. (2013).

But we still want to know what the BLR gas is actually doing and, in the process, make more accurate mass measurements. Moreover, we’d really like to get reverberation measurements for the strong UV lines, like Ly α λ1215 and C IV λ1549: much of the BLR emission is in these lines and we know from several International Ultraviolet Explorer (IUE) reverberation programs from over 20 years ago that the lags for the UV lines are about half those of the hydrogen Balmer lines in the optical, so they probe a different part of the BLR. The IUE data were ground-breaking, but not high-enough quality to determine the structure and kinematics of the BLR, only the mean lags. That would require Hubble and its superb spectrometers.

Hubble time is hard to get, especially if you need a lot of it. We knew that we’d need a really compelling science proposal and a seamless technical case for the large allocation to do a reverberation program right. We started out assuming that a realizable cadence would be one observation per day, that a single visit must yield spectra of the required quality in a single orbit, and that the program would have to be completed with no significant gaps in one observing season. This put constraints on the luminosity of the AGN (since the BLR size depends on it), its apparent brightness, and its location on the sky. We further desired a target AGN that was previously well-studied so we could avoid AGNs where the UV emission lines were strongly self-absorbed and so we could accurately model its behavior to determine how many orbits we would actually require – the number of orbits was the most difficult parameter to pin down, since sometimes AGN flux variations behave in ways favorable for a reverberation-mapping experiment, and sometimes they don’t. Our success rate on the ground is typically around 60% or so, so this is kind of a high-risk business. After lots of simulations, we determined that NGC 5548 was the best target and that our best estimate of the required number of visits was 180. None of our simulations succeeded with fewer than 100 visits, about half succeeded with 150 visits, but all of them succeeded with 180 visits.

We first submitted this proposal in Cycle 12 in 2003. We were finally awarded the time to carry this out in Cycle 21 – this is either a case study in perseverance or obsession, I’m still not sure which. It was a challenging program to schedule and execute, but the schedulers did a wonderful job, and we wound up with 171 epochs with only a few one or two-day gaps due to safing events, against which our program was robust, as anticipated in our simulations. We had to deal with some complications, such as moving to different positions on the detector to avoid depletion by geocoronal Ly α, but this only complicated the data reduction and didn’t adversely affect the final results. A major amount of work went into completely recalibrating the Cosmic Origins Spectrograph because our data-quality requirements exceeded specifications of the standard pipeline reduction.

The final light curves are beautiful (Figure 5), although some of the behavior was surprising, even in our initial quick-looks based on standard pipeline reduction. For the first 60 days of the program, things looked nominal – the emission-line light curves look like a smoothed and time-shifted version of the continuum light curve, though the time shifts (or lags) are shorter than we expected. After this, the emission lines behavior began to deviate from the expected linear response in a complicated way.

Figure 5. Light curves based on HST COS spectra obtained in the AGN STORM project. The top panel shows the continuum variations and the lower panels show the light curves for Ly α, Si IV λ1400, C IV λ1549, and He II λ1640. From De Rosa et al. (2015).

Equally disturbing was the fact that the UV resonance lines were strongly absorbed (Figures 6 and 7). Recall that one criterion for target selection was weak or absent absorption in the emission lines. While narrow absorption features had previously been detected in NGC 5548, a combined XMM/HST campaign the previous year (Kaastra et al. 2014) had shown strong and variable broad absorption for the first time (Figure 8). The broad absorption weakened toward the end of the 2013 campaign, and all we could do at that point was hope that trend would continue into 2014. Our first spectra in early 2014 showed, however, that variable broad absorption was still present, though weaker than in 2013. This added another layer of complexity to the analysis.

Figure 6. The top panel shows the mean C IV profile during the AGN STORM program. Note the strong narrow and broad absorption features shortward of line center. The middle panel shows the rms residual profile, which isolates the variable part of the emission line. The bottom panel shows the mean reverberation lag in each velocity bin. In all cases, black is for the entire campaign, gray is for the first half, and orange is for the second half. From De Rosa et al. (2015).

Figure 7. The mean, rms, and reverberation lag profiles as in Figure 6, but for Ly α. The broad (damped) absorption shortward of the broad emission line is due to interstellar absorption in our own Galaxy and the narrow emission superposed on it is geocoronal Ly α emission.

Figure 8. Historical C IV profiles for NGC 5548. The cyan profile from 1993 shows no broad absorption. The black profile is from AGN STORM and shows weak broad absorption compared to what was observed a year earlier (green, red, and blue profiles) by Kaastra et al. (2014). Figure courtesy of G. Kriss.

The data product that we most desired is a projection of the BLR kinematics and velocity field into the two observable parameters, Doppler velocity and time delay (Figures 9 and 10). This “velocity–delay map” is essentially the observed response of the emission lines to an instantaneous (“delta function”) outburst by the continuum source. Recovery of the velocity–delay maps for the various emission lines was complicated by the non-linear emission-line response during much of the campaign and by the strong broad absorption features. Nevertheless, we were able to recover velocity-delay maps for the three strongest lines, Ly α, C IV, and H β. All of them show the signature of an inclined disk with a fairly sharp outer boundary, though the response of the far side of the disk is surprisingly weak. The weak response of the far side suggests that fewer ionizing photons are reaching the far side than the near side: this might also explain the surprisingly small lags (since mostly we’re seeing the response of the near side) and the anomalously small equivalent widths of the lines (i.e., the emission lines are weak compared to the continuum).

Figure 9. Preliminary UV velocity-delay map based on AGN STORM data. The upper left panel is the velocity-delay map for Ly α + NV, Si IV, C IV, and He II; the orange dashed ellipses trace the faint disk signature for a mass of 6 × 107 solar masses at an inclination of 50°. The lower left panel shows the variable part of the line profile: the average for all time delays is in black, and the averages for binned lags of 0-5 days, 5-10 days, 10-15 days, and 15-20 days is shown in blue, green, orange, and red, respectively. The upper right panel shows the “delay-map” (i.e., integrated over all velocities) for Ly α, Si IV, C IV, and He II in red, orange, green, and blue, respectively, and in black for the entire spectrum. Figure courtesy of K. Horne.

Figure 10. Preliminary velocity-delay map for He II λ4686 and Hb λ4861 from AGN STORM optical spectra. Panels are as in Figure 9. In the upper right panel, He II is shown in blue, H β is in red, and the core of H β is in orange. Based on data from Pei et al. (2016). Figure courtesy of K. Horne.

The latter two points are things we know because NGC 5548 is such a well-studied AGN: there have been almost 20 reverberation campaigns – mostly ground-based optical abut two UV campaigns, one involving HST – that included this source. NGC 5548 is essentially a “control” object in the sense that while some properties of this AGN are expected to change over timescales long compared to reverberation (luminosity, BLR radius), others are not (black hole mass, inclination) – if reverberation-mapping is working as it should, we should get the same mass every time. Because we have this wealth of archival data, we could tell that something odd was happening in NGC 5548 rather than erroneously conclude that NGC 5548 is simply an odd source.

So what exactly is going on with NGC 5548? A couple things. First, we find that the narrow absorption lines are varying. This provides a strong diagnostic of the unobservable ionizing continuum as each line responds to the continuum at the ionization energy of the relevant ion. As the continuum at the ionizing energy increases, the ionization level increases so the line becomes weaker. For example, singly-ionized silicon has an ionization potential of 16.3 eV, so when the continuum at 16.3 eV (~760 Å) increases, more of the silicon becomes doubly ionized and the equivalent width of Si II λ1526, which arises from singly ionized silicon, decreases. However, this pattern is broken after the first 60 days of the campaign. The lower-ionization absorption lines are still following the pattern, but the higher ionization lines are not responding anymore. While the continuum that drives the variability of the broad Balmer emission lines (just shortward of 912 Å) is still varying with the observable continuum (~1150 Å), the higher energy continuum (at wavelengths shorter than, say, ~500 Å) is not. So at least part of the reason the emission line response is changing is because the shape of the ionizing spectrum has changed. As an aside, we were also able to determine where the narrow absorption arises, based on the “recombination time”, i.e., the timescale to return to the lower ionization state when the continuum becomes faint again. The narrow absorption arises ~1 – 3 pc from the black hole, in the same gas that produces the [O III] λλ4959, 5007 emission lines seen prominently in the optical spectrum (Peterson et al. 2013).

Second, the broad absorption is also present, but weaker than it was in 2013 (Figure 8). While we don’t know where the broad absorption arises, it’s likely that it occurs on the BLR scale. It also stands to reason that if there are absorbers along our line of sight that there are absorbers along other sight lines as well. We can speculate that, in fact, there is very heavy absorption between the accretion disk and the far side of the BLR, which would account for the weakness of the emission lines, the unexpectedly short lags, and the faint response of the far side seen in the velocity–delay maps.

I’ve focused this discussion almost entirely on the BLR because that was the original goal of the experiment. Our preliminary analysis confirms the black hole masses that we’ve estimated from the simpler sort of reverberation analysis described earlier. We’ve learned that the BLR in NGC 5548 is at least in part a disk seen at moderate inclination, and we’ve concluded that there is a lot of absorption on different scales – anticipation of the importance of strong variable absorption was the omission in our original simulations, simply because we didn’t expect it to be a factor. The presence of absorbing gas has complicated the analysis, revealing a richer, more complex environment than we’d anticipated. While we think we have the basic ingredients now, we’ve still got a lot of detailed modeling to do. So far, the AGN STORM project has produced 6 papers (see references below) and several more are in preparation.

Some of the more important things we found had to do with the accretion disk itself, and I’ll have more to say about that another time.

This Month’s Featured Author

Dr. Brian Williams received his B.S. from Florida State University in 2004 and his Ph.D. from North Carolina State University in 2010. He was a NASA Postdoctoral Fellow at NASA Goddard Space Flight Center for three years, after which he worked as a research scientist at NASA GSFC with Universities Space Research Association. He arrived at STScI in February of 2017, and is currently a Support Scientist in the Science Mission Office. His research interests include supernovae and supernova remnants, shock physics and particle acceleration, and dust in the interstellar medium.